Radiation detectors, which may be X-ray detectors, infrared radiation detectors or other types, commonly have an evacuated envelope with a vacuum space, at least one radiation detector element being mounted in the vacuum space. These devices include scanning electron microscopes (SEM), X-ray spectrometers, infrared spectrometers, and other known devices. For optimal operation, with low electronic noise and high sensitivity, these detectors are operated cryogenically. Therefore, the detector is provided with a cooling device. In order to reduce parasitic heat input to the system, the detector and cooling system are generally insulated from the environment in a Dewar, having a vacuum space within a sealed envelope or chamber. A cooling element is provided in the chamber and serves to cool the detector element during operation of the detector. The vacuum minimized heat conduction through a gas-filled space, and the surfaces are formed with low heat radiation emissivity construction. In general, the detector may be allowed to return to ambient temperature when not in operation.
A member, in thermal communication with the cooling device, protrudes into the chamber and supports the detector. As is known, the member is actively cooled by the Joule-Thomson effect (gas expansion), liquid nitrogen (or other gas), Stirling cycle cooling, Peltier junctions, or by other known means. The cooled protruding member is often termed "the cold finger" of the detector. The cold finger is also thermally coupled to a detector to be cooled, and generally acts as a mechanical support as well.
In order to achieve the desired cryogenic temperatures, around 87K at the junction of the detector holder and cold finger for lithium drifted silicon X-ray detectors, and low vibration, the Dewar is typically filled with liquid nitrogen, which exists at 77K in the liquid state at standard pressure, which is allowed to vaporize, providing cooling of the cold finger in the cold-finger assembly. The radiation detector is thermally conductively attached to an end of the cold finger opposite the cryogenically cooled end. The finger is thus insulated from ambient atmosphere and contained within a housing. The vaporization of liquid nitrogen in a Dewar to achieve cryogenic temperatures is an inherently low vibration-producing action, and therefore liquid nitrogen is typically employed to cool vibration sensitive instruments. Liquid nitrogen is expensive, requires frequent refilling of an internal supply in the detector, and is often delivered in bulk from off-site, requiring scheduled service. Further, the handling of liquid nitrogen may be hazardous and undesirable in certain environments, such as clean rooms and remote locations.
It is known that a significant cause of detector failure is the gradual degradation of the vacuum in the evacuated space due to, e.g., internal out-gassing of the various component parts of the detector exposed to the vacuum and leakage from outside the evacuated envelope. In order to reduce outgassing of the component parts of the detector in the vacuum space during the service life of the detector, these component parts are generally prebaked in known manner in vacuo before assembly, and a general bakeout of the assembly is also carried out before mounting the detector electronic element(s). The excessive outgassing which generally occurs in X-ray and infrared detectors may be due to the fact that the gases cannot be driven out by baking the whole device during vacuum pumping (in the way which is usual for other vacuum devices) because X-ray or infrared detector elements are damaged at high temperatures.
This degeneration in the vacuum eventually leads to the situation in which a cooling element is no longer able (at least in an efficient manner or sufficiently fast) to cool the detector element to the desired temperature for efficient detection of the radiation. Thus, the detector lifetime is curtailed, especially as only limited cooling power is available to cool the detector. Furthermore, the outgassing into the vacuum space provides a significant thermal transfer path between the cold finger and the outside of the detector, when the pressure exceeds 1.sub.-- 10.sup.-3 Torr. In mechanical cooling systems, the thermal capacity drops as the temperature differential increases, so that with an insufficient initial vacuum, necessary operating temperatures may never be reached.
The cooling system is generally not provided with an extraordinarily large cooling capacity because this may introduce increased vibration, leads to inefficiency and increased size, and may be difficult to control. In operation, the desired vacuum condition in the housing reduces the heat transfer from outside the vacuum space, thus limiting the amount of heat which must be removed through the cold finger. As the vacuum deteriorates, heat transfer from outside the space increases, imposing a greater heat load on the cryogenic system. Thus, the detector lifetime may be curtailed.
U.S. Pat. No. 3,851,173 describes one example of an infrared detector comprising an envelope, a cold finger in the envelope and at least one detector element mounted on an end of the cold finger so as to be cooled by the cold finger in operation of the detector. The envelope has an outer wall extending around the cold finger and an infrared-transmissive window facing the detector-element end of the cold finger. A space is present between the cold finger and the window and outer wall of the envelope, and a chemically-active getter is present in said space to getter gases from the space. The envelope of U.S. Pat. No. 3,851,173 is a vacuum Dewar having a vacuum space between inner and outer walls of the Dewar envelope. The inner wall defines an inner chamber accommodating a cryogenic cooling element which serves to cool the inner wall end and hence the detector element thereon, during operation of the detector. The cooled inner wall forms the cold finger of the detector. A major cause of infrared detector failure in designs of this type is the gradual degeneration of the vacuum in the space between the inner and outer walls due to internal out-gassing of the various component parts of the detector exposed to the vacuum. In the detector of U.S. Pat. No. 3,851,173 a non-evaporable chemically-active SAES getter unit is mounted in the outer wall to maintain a good vacuum in the space between the outer wall and the cold finger.
Another example of an infra-red radiation detector incorporating a getter to maintain a vacuum in a Dewar is described in U.S. Pat. No. 3,786,269. Its detector element array is cooled by a Stirling cycle refrigerator at approximately 50.degree. K. In this particular detector a series of chemically active getters are mounted around the outer perimeter of the outer wall and protrude through into the vacuum space between the outer wall and the cold finger. In order to getter sufficiently large quantities of gas, such chemically active getters need to have a large surface area and are bulky; this can present a dimensional size problem in the spacing of the inner and outer walls and/or the shape of the outer wall.
Such chemical getters as employed in U.S. Pat. Nos. 3,851,173 and 3,786,269 are activated by being taken to a high temperature (for example 900.degree. to 1,000.degree. C.) during or after evacuating and sealing the Dewar envelope. This is normally achieved with an electrical heating element embedded in the getter material formed as a unit with electrical connection leads (not specifically shown in U.S. Pat. No. 3,851,173) passing through vacuum-tight seals in the Dewar. U.S. Pat. No. 4,206,354 shows an example of such a Dewar getter with its connection leads. For this reason the getter is mounted in the outer envelope wall with external electrical connections, and a large spacing is required between this type of getter and the detector element which could otherwise be damaged by the very high temperature. These factors lead to an increased size for the Dewar envelope and even the adoption of unconventional Dewar envelope outlines.
U.S. Pat. No. 5,177,364, incorporated herein by reference, discloses a cryogenic infrared detector system having a chemically-active getter present in an evacuated space to getter gases from the space. The getter comprises a porous layer of chemically-active getter material deposited on an inside surface area of the outer wall at a location separated from both the cold finger and the window. United Kingdom patent application GB-A-2,179,785 describes a pumping tubulation getter device for an electron discharge device such as a ring laser gyroscope. GB-A-2,179,785 describes the provision of the getter as an electrophoretically-deposited layer of porous sintered non-evaporable getter material selectively deposited on the internal surface of the pump tube through which the device is evacuated. Getter-free zones are present at each end on the internal surface of the tube. The getter is activated by HF induction heating of the tube. By locating the getter within the pump tube, space problems which otherwise arise in trying to accommodate a getter unit within the device chamber are avoided.
U.S. Pat. No. 5,235,817, incorporated herein by reference, discloses a radiation detector cryogenic cooling apparatus having a plurality of nested space thermally conductive elongated members having first and second ends. These nested tubes reduce thermal radiation parasitic input to the detector and cold finger, allowing higher efficiency and lower detector temperatures to be achieved. U.S. Pat. No. 5,274,237 relates to a cryogenically cooled radiation detector having means for preventing icing.
The use of molecular-sorbent porous bodies as getters is known. See, e.g., "Zeolite and Molecular Sieves" by D. W. Breck, John Wiley and Sons, Inc., New York, London and Sydney (1974) for a general background description of such porous molecular sorbents. It is known to use molecular-sieve getters in the form of loose beads or loose pellets retained behind a screen or in a cage. See, GB-A-921,273 which relates to liquefied gas storage containers and GB-A-1,192,897 which relates to circuit breakers. Molecular-sorbent getters are known to have increased sorption efficiency at cryogenic temperatures, unlike chemically active getters, which have increased sorption affinity at higher temperatures, e.g., about 300.degree. K. and above.
Activated carbon molecular sorbent getter is known. See, e.g., Norit.RTM. activated carbon, Product Information Bulletin No. 206 (Rev. 1-91), from American Norit Company, Inc., incorporated herein by reference.
Activated carbon may be provided as granules or in shaped forms. Preshaped forms are provided either by pyrolizing a preformed organic material, which may be a polymer or organic mass, or by providing a binder for previously activated carbon powder. During pyrolysis, a properly selected organic material will undergo a predictable dimensional alteration; therefore a preformed organic material may be provided to pyrolize into the desired shape.
The getter of U.S. Pat No. 4,474,036 is, for example, a zeolite or synthetic zeolite material. The getter may also be a molecular sorbent material formed in situ, minimizing the need for adhesives, such as silica gel. This material is formed into a shape which conforms to a cooled portion of the vacuum space, such as around the cold finger, and is preferably not a loose granulate, because thermal transfer through such a material is retarded, and firm, thermally transmissive contact with the cooled portion of the Dewar cannot be assured. Therefore, the getter is generally provided as a formed element in substantial thermal contact with a cooled portion of the Dewar, such as by a low-outgassing epoxy. The epoxy may be filled with a thermally conductive material, such as silver powder. The getter is generally very fragile so that it is preferably supported by other structures. It is considered desirable in this application to effect reduction of the pressure in the evacuated space to less than about 1.sub.-- 10.sup.-3 Torr in less than about 30 seconds after the time the getter is cooled.
Typically, the synthetic zeolite body of U.S. Pat. No. 4,474,036 is composed of particles having a width of at most a few micrometers and with somewhat irregular inter-particle voids also in the body. The pores of the porous zeolite particles forming the body have a width comparable to molecule sizes (up to approximately 0.5 nm) of gases in the vacuum space and were formed by driving off the water of crystallization of the zeolite material before molding the zeolite particles together in an annular shape to form the body; the heating required to effect this dehydration is thus performed before mounting the getter in the Dewar envelope. The resulting molecular-size pores permeate the zeolite particles to give an extremely large internal surface area, as a result of which the cooled body can absorb a large volume of gas by adsorption on the inner surfaces of the pores.
Since the cooling element of U.S. Pat. No. 4,474,036 is provided to cool to only a moderate cryogenic temperature, the good large-area thermal contact between the inner major surface of the getter body and the surface of the Dewar is particularly important in efficiently cooling the molecular-sieve body, i.e., the molecular sorbent getter, to obtain a high sorption efficiency. An annular configuration for both the getter body and the cooled surface also minimizes the amount of epoxy adhesive necessary to secure the getter body to the surface; this is important since a large amount of epoxy can increase out-gassing into the vacuum space. In a particular example the epoxy film may be typically 100 micrometers thick.
Molecular-sorbent getters do not require activation heating to very high temperatures after mounting in a vacuum space, so that the getter can be mounted in the proximity of the detector element so as to obtain maximum cooling of the molecular-sorbent porous body. A most efficient cooling of the shaped molecular-sorbent porous getter body can be achieved when the body is mounted around the inner wall of the detector Dewar in a vicinity where the inner wall is directly cooled by the cooling element. The shaped getter body or bodies of molecular-sorbent porous material may be bonded to an outer surface of the inner wall, and/or any other cooled surface associated with the inner wall. Thus, an annular radiation shield may be mounted at the end of the inner wall around the detector element, and the cooled surface to which at least one said shaped getter body is secured may be an outer surface of the radiation shield.
These molecular sorbent getters may be employed to reduce the effect of internal out-gassing in infrared detectors. Thus, it is known to provide at least one molecular-sorbent getter in a vacuum space of a housing for gettering gas molecules from this space. According to U.S. Pat No. 4,474,036, incorporated herein by reference, having an infra-red radiation detector mounted in an evacuated space with an infrared transmissive window, and cooled by a cold finger. U.S. Pat. No. 4,474,036 discloses the use of Joule-Thomson effect cooling for the cryogenic infrared detector system, e.g., by allowing a gas, such as dry air, nitrogen or argon, to expand in an area of lower pressure, absorbing heat. See also EP-A-0,006,297. In general, infrared detectors of the type employed in U.S. Pat. No. 4,474,036 are not especially sensitive to vibration and therefore the intrinsic vibrations from a Joule-Thomson cooling system did not require redress.
Cryogenic cooling apparatuses for cooling other types of radiation detectors to cryogenic temperatures are also known. For example, a radiation detector is employed with an electron microscope for detecting X-rays incident on a specimen being spectroscopically examined. Such X-ray detectors do not require an optically transparent window. The specimen is placed within the microscope and receives incident electron bombardment from the microscope. Scattered radiation from the specimen is then detected by a cryogenically cooled detector which converts the radiation to an electrical signal in a known manner for spectroscopic analysis. The detector is mounted on an elongated structure referred to in the art as a cold finger, extending to a position where it is desired to detect X-rays. The finger is cantilevered to a support so as to be placed within the region of the electron microscope adjacent to the specimen. The interior of the microscope and the region surrounding the cold finger are within an evacuated chamber. Cooling of the detector is accomplished by the finger which is thermally conductively connected to a source of cryogenic cooling, for example, a Dewar containing liquid nitrogen.
U.S. Pat. No. 5,337,572, incorporated herein by reference, relates to a closed cycle cryogenic refrigerator. This system employs a ternary mixture of gasses to allow a single stage compressor to achieve cooling temperatures of between about 65 and 150K, when used with a Joule-Thomson cryostat. U.S. Pat. No. 5,313,801 relates to a throttle valve for a cryostat, allowing automatic internal temperature regulation with a minimum of moving parts.
A Joule-Thomson cryostat operates by allowing an isenthalpic expansion of a medium, thereby cooling the medium. Therefore, a compressed refrigerant is provided to a flow restricting orifice. An expansion chamber is provided after the orifice, having a greater cross sectional area than the orifice. In general, a turbulent process occurs in the expansion chamber, producing vibration. Such a cryostat may operate with a liquified refrigerant, especially to obtain cryogenic temperatures. Such liquids generally expand supersonically and turbulently at the flow restricting orifice.
U.S. Pat. No. 4,910,399 discloses an electron microscope with an X-ray detector in an arrangement as described above. U.S. Pat. No. 3,864,570 also disclose an X-ray detector for use with an electron beam producing device disclosing a cold finger structure. British Pat. No. GB 2,192,091 discloses a still further embodiment of an electron microscope and cryogenically cooled X-ray detector system.
Typically in these kinds of systems, it is known to reduce heat input to the cold finger mounting the X-ray detector by using low emissivity warm surfaces and by wrapping the cold finger with low emissivity aluminized Mylar.RTM.. The heat to be extracted from the system comes from four sources. First, the X-ray detector system, including the detector crystal and electronics, generates an amount of heat in operation. Second, gas molecules in the evacuated space conduct heat to the detector and cold finger. Third, mechanical support structures bridging the cooled elements, i.e., the cryogenic cooler, the cold finger, and the detector assembly, and warm structures, i.e., the outer housing, conduct heat. Fourth, energy is radiated to the detector and cold finger, both heat energy from warm surfaces and energy from the operation of the apparatus, a portion of which is absorbed. At steady state temperature, the cold finger conducts the heat input along its length to the cooling system, and there is a temperature gradient between the radiation detector at the end of the cold finger and the heat sink of the cooling system. Care must be taken to minimize this gradient for acceptable performance. Additionally, the Mylar.RTM. and other organic compounds used in the insulation system present in the evacuated chamber in which the cold finger is secured may evolve contaminants undesirable when used in a ultra-high vacuum (UHV) environment.
In an electron microscope, a cavity of the electron microscope receives a specimen being examined by an electron beam produced by the microscope. The beam, typically less that 1 micron diameter, is incident on the specimen, producing X-rays which are then radiated from the specimen. The detector is placed within the microscope cavity adjacent to the specimen and detects the radiation emanating from the specimen. The detector, which includes a semiconductor detector for converting the X-ray signals to electrical impulses and a field effect transistor (FET) for sensing and amplifying the detected signal, produces an electrical signal which is passed to an external electronic circuit for analysis.
For example, one such X-ray detector system is disclosed in U.S. Pat. No. 4,931,650. In this environment the known detector comprises a semiconductor mounted at the end of the cold finger or probe introduced into the microscope close to the specimen. The cold finger is surrounded by an envelope and a vacuum is maintained between the finger and the envelope. The cavity in the microscope receiving the cold finger is also held at a vacuum. According to U.S. Pat. No. 4,931,650, a problem with X-ray detection in this apparatus is that it is sensitive to contamination and, especially, ice buildup. Moisture tends to accumulate on the detector, decreasing its effectiveness. This moisture may be removed and the performance of the detector improved by a warming-up procedure. Generally, prior art systems require that the system be disconnected for a period of time usually every few days or, in some cases, hours, so as to warm up the system and remove the accumulated moisture. The warming-up procedure involves pumping the detector to maintain a vacuum while removing water vapor as it evaporates. Such a procedure is used only as part of a major overhaul involving the return of the detector to the manufacturer. For windowless detectors, a warming-up procedure may involve using the vacuum pumping system of the electron microscope, which must maintain a vacuum during operation. In a windowed device for spectroscopy type examination, the cold finger is permanently maintained in its own evacuated housing. With liquid nitrogen cooling and sufficient getter material, an acceptable vacuum may be maintained for years.
In U.S. Pat. No. 4,886,240 a non-evacuated Dewar is disclosed which employs a molecular sieve that serves to absorb gases in the Dewar when cooled, for operation of a detector and to prevent liquid formation onto the detector. A desiccant also may be used to absorb moisture. The molecular sieve is employed for removing gases from the area adjacent to the detector when operating. Fluid contained within the cold finger expands, thereby absorbing thermal energy. The Dewar housing is back filled with inert gas such as nitrogen. This gas is at one atmosphere, e.g., at atmospheric pressure.
Known systems require the use of liquid nitrogen to achieve sufficiently low temperatures for high performance operation, e.g., cryogenic temperatures with low vibration for high resolution. Liquid nitrogen, however, is undesirable in certain applications, such as semiconductor clean rooms and remote lab sites. Peltier junction (thermoelectric) coolers have great difficulty in achieving sufficiently low temperatures for high performance operation. See, U.S. Pat. No. 5,075,555, which relates to a Peltier cooled lithium drifted X-ray spectrometer. Mechanical cooling systems, such as Stirling cycle refrigerators require complex mechanisms near the cold finger and may introduce vibrations.
Systems are known which attempt to actively damp vibrations. See Garba et al., "Piezoelectric Actuators on a Cold Finger", Technical Support Package, NASA Tech Brief 19(1) item 277, JPL New Technology Report NPO-19090, incorporated herein by reference. Such systems require at least one actuator for each axis of compensation and either a known predetermined vibration pattern or sensors to determine the vibration to be damped.
U.S. Pat. No. 5,225,677 relates to a protective coating for an X-ray detector. U.S. Pat. No. 5,268,578 relates to a specially shaped X-ray detector.